1. Field of the Invention
The invention relates to the fields of ceramics and laser technology and relates to a method for producing by laser gastight and high-temperature resistant connections of shaped parts made of a non-oxidic ceramic, with which, e.g., low-melting-point or radioactive materials can be enclosed in a container made of ceramic.
2. Discussion of Background Information
Welding and soldering methods for joining ceramic and, in particular, heavy-duty ceramics are known (Hesse, A. et al., Keramische Zeitschrift 3 (1994), pp. 147-150; Boretius, M. et al., VDI-Berichte, Volume 670, pp. 699-713, VDI-Verlag, Düsseldorf, 1988).
With these integral joining methods, soldering stands out compared to (diffusion) welding because of lower technological complexity and higher reproducibility and reliability.
Active soldering with metallic solders produces relatively solid bonds. The use of this method with PVD-metallized or laser-treated ceramic achieves, in particular, favorable wetting and flow properties of the solder (Wielage, B. et al., VDI-Berichte, Volume 883, pp. 117-136, VDI-Verlag, Düsseldorf, 1991). With this method, the ceramic is metallized and subsequently brought in contact with the solder in an oven. The temperature is thereby raised above the melting temperature of the solder. In the molten state, the solders wet the metallized ceramic surface and form a solid bond after cooling.
However, in addition to the use of metallic solders, glass/ceramic solders can also be used to join ceramic (Boretius, M. et al., VDI-Verlag, Düsseldorf, 1995). With this method, the workpiece is inserted into an oven and heated to above the melting temperature of the solder while an external force is applied to press together the two parts to be joined. The force is necessary to achieve a sufficient degree of compression in the joint seam and gas tightness. The method in the oven thereby takes place in a protective gas atmosphere or in a vacuum. After reaching its melting temperature, the solder forms a liquid phase which wets the surfaces to be joined and leads to a solid ceramic bond after resolidification. These solders are used, in particular, if there are higher requirements in terms of corrosion resistance and high-temperature stability, but lower requirements in terms of the transmission of mechanical forces.
Glass/ceramic solders wet ceramics well and, in contrast to metallic solders, can thus be used without metallization of the ceramic surface. A further advantage of glass/ceramic solders is the fact that they are gastight. Crystalline glass/ceramic solders are converted into a ceramic, polycrystalline state after solidification. The soldering temperature mostly corresponds to the service temperature. The particular advantage of glass/ceramic solders is the fact that they render possible an adjustment of the coefficient of expansion and the temperature stability of the joint area.
A drawback of this method is that the workpieces have to be adapted to the respective oven with regard to their size, and that materials located inside a ceramic container that is to be sealed by joining are also exposed to high stresses because of the relatively long and high temperature stress. Low-melting-point materials cannot be sealed in such ceramic containers with these technologies.
Furthermore, methods are known for joining ceramics with solders to non-high-temperature-resistant ceramic joints by using a laser, whereby only small components are joined and a vacuum or a protective gas is required as well (Harrison S, et al., Solid Freeform Fabrication Proceedings, Proc. of the SFF Symp. Austin, U.S.A., Aug. 10-12, 1998, (1998) pp. 537-542).